The present invention relates to a process for converting coal directly into predominantly gaseous to liquid products suitable for making hydrocarbon fuel by digesting the coal in a particulate, more particularly comminuted condition at elevated pressure and temperature, preferably under hydrogenation conditions, more particularly in the presence of hydrogen under pressure, slurried in a solvent or pasting oil for coal, and wherein the solvent or pasting oil includes recycled heavy bottoms fraction and a recycled lower boiling fraction.
Several such processes are known. In order to attain satisfactory depths of extraction it is usually preferred to employ hydrogenative conditions. Such hydrogenative conditions may partly or wholly be created by the use of a solvent having pronounced hydrogen donor properties, but are usually due at least in part to the employment of hydrogen under pressure, with or without an extraneously introduced hydrogenation catalyst.
Since Bergius hydrogenated coil to oil in 1911, a process requiring very high pressures, several coal liquefaction processes have been developed to at least a continuous bench unit stage and many more such processes have been proposed. The latter have mostly been based on results obtained in batch autoclave experiments.
Although the production units constructed and run in Europe, notably in Germany, before and during World War II were technically successful, the very high pressures and temperatures that these units used made them uneconomic in any peace-time economy where there is a competitive market (W. R. K. Wu and H. H. Storch, Bureau of Mines Bulletin 633, 1968, p 6).
Today there is considerable incentive to develop a coal conversion process that would successfully compete with petroleum as a source of conventional distillate fuels. A prime prerequisite for lowering the cost of coal hydrogenation, or more accurately, hydrocracking, is a relatively low pressure process in which hydrocracking would proceed rapidly enough to yield distillable liquid hydrocarbons as the main product. The process developed by Hydrocarbons Research Inc., termed the H-Coal process, is an important advance in this direction both technologically and economically (see "Present Status of the H-Coal Process" by C. A. J. Johnson et al, Clean Fuels from Coal Symposium II, Institute of Gas Technology, Chicago, June 1975). The H-Coal process uses a supported Co-Mo catalyst in an ebullating bed reactor at pressures around 200 bar and temperatures above 400.degree. C. It can produce as main product either socalled "syncrude", i.e. a product resembling crude oil, yielding various distillates and some, e.g. about 10% undistillables, or a partially distillable heavy furnace oil. The catalyst is fouled relatively rapidly and fresh catalyst has to be added to and used catalyst withdrawn from the reactor at intervals. Coal is introduced to the reactor in the form of a slurry with a heavy oil fraction.
In contrast to the H-Coal process, processes being developed by the Gulf Research and Development Company to produce distillates from coal slurries make use of fixed catalyst beds.
In one instance (S. W. Chun, D. C. Cronauer and T. W. Leslie, U.S. Pat. No. 3,957,619) a special reactor with free segments interspersed with catalyst-packed segments is advocated for coal hydrocracking. In another, a coal solvent slurry is passed through a packed catalyst bed in which the void volume is large (A. A. Montagna and H. G. McIlvried U.S. Pat. No. 3,997,426). The Gulf R and D Company has also developed a coal liquefaction process in which the ash in the coal is used as hydroliquefaction catalyst. Here a solvent-coal slurry, the solvent being a fraction having roughly a boiling range of 200.degree.-450.degree. C. at atmospheric pressure, is fed to the reactor. The process is completely self-sustaining in terms of solvent and the product varies from a solid low-ash low-sulphur SRC (solvent refined coal) to a heavy partially distillable heavy fuel oil (G. R. Pastor and C. H. Wright, U.S. Pat. Nos. 3,892,654; C. R. Hinderliter and R. E. Perrussel, 3,884,796; W. C. Bull, C. H. Wright and G. R. Pastor, 3,884,794; C. H. Wright and G. R. Pastor, 3,884,795).
In accordance with U.S. Pat. No. 3,726,785 the slurrying of coal in two-coal-derived solvent fractions, a light and a heavy fraction is described. According to that patent the two solvents are to be used separately, and the combination of the two solvents in a single solvent system is advised against.
Johanson and Wolk have in a recent patent (U.S. Pat. No. 4,045,329) shown that when heavy residual oils, e.g. those boiling above 427.degree. C. at 1 atm., are recycled to the H-Coal reactor, it is advantageous to introduce into the reactor simultaneously a certain amount of distillate boiling in the range 232.degree.-316.degree. C. at 1 atm. The preferred quantity of the recycled 232.degree.-316.degree. C. fraction is 5-25% of the total recycled oil and its function is to control the viscosity of the liquid in the ebullating reactor, thus providing adequate hydrocracking conditions for breaking the +427.degree. C. fraction down to distillate in the 204.degree.-427.degree. C. boiling range.
The product spectrum of this process extends fairly uniformly across the whole range from methane gas to the top of the diesel oil range. The yield of gaseous products is substantial. The total diesel oil yield represents less than half of the total product yield and is usually less than half of the total yield of distillate products. Part of the total yield (typically about 9%) is unavoidably in the form of undistillable residue.
The "ebullated" bed concept and the use of a catalyst are essential features of the process disclosed in U.S. Pat. No. 4,045,329.
The use of light solvents above their critical temperatures is known in coal liquefaction. Of especial relevance are patents by D. B. Urquhart (SA 75/6634 and U.S. Pat. No. 4,019,975) in which it is shown how coal can be partially liquefied to a main product which is either distillate or a tar-like residue, depending on whether or not a catalyst is used. In both cases, however, roughly 50% of the coal was reported as unreacted.
Moreover, in the claimed process of the Urquhart invention the only solvent used for slurrying is a light fraction such as toluene. The same inventor has also reported that liquefied coal can be separated from unreacted coal in a supercritical separation step, wherein the liquefied coal effectively distils with the supercritical solvent under carefully adjusted conditions, leaving behind the unreacted coal, including the inorganic ash. Such a light solvent can have an advantageous effect on the mass transfer of hydrogen to the coal molecules undergoing cracking and hydrogenation. However, the light solvent suffers from the severe disadvantage that it has limited coal carrying capacity through, in particular, the preheaters of a continuously operating liquefaction unit. Coal tends to settle in the preheater tubes despite high linear velocities of gas (hydrogen or hydrogen-containing) and solvent coal slurries, even when the coal is ground to below 200 mesh. Coal deposition in the preheaters is highly undesirable since it forms blockages which can rapidly harden. To summarise then, difficulties have been experienced in prior art processes to achieve adequate self-sufficiency in respect of solvent requirements and/or adequate solids carrying capacities of the solvent. Solvents which do have a desirable solid carrying capacity tend to result in net products mainly consisting of solid or near solid products, there being little or no net product in the highly desirable liquid hydrocarbon region. Increasing the hydrogenation rate, either in order to improve the depth of extraction or to increase the yield of liquids has sometimes tended to result in excessive gas yields. There exists a need in the art for a convenient process which is capable of producing high extract yields, particularly in the liquid hydrocarbon range, more particularly the diesel fuel range with low to modest hydrogen consumption, very high pressures being avoided. At the same time the main products should be distillates which can be refined to conventional liquid fuels and chemicals without undue difficulty.
Preferred embodiments of the present invention are directed to processes, capable of converting substantially all the liquefiable coal components to distillate products, whilst being capable optionally to be so modified that a non-distillable residue is formed as a valuable byproduct having surprisingly superior characteristics as a raw material for making premium electrode coke.
Preferred embodiments of the present invention are furthermore directed to processes yielding more than 50% products in the diesel oil range based on total distillable products and preferably even when based on the total range of all products, more particularly as much as 60% or more based on dry ash-free coal.